Abstract: In order to gain a deeper understanding of the mechanism of the effect of CO₂ and N₂ injection in fractured-vuggy reservoirs on the residual oil in the filling medium inside the fractures and pores, pressure depletion experiment was conducted to determine the gas diffusion coefficient and solubility, for the purpose of studying the effect of water saturation on the diffusion pressure, solubility and diffusion coefficient of the two injected gases and analyzing the mechanism of the effect of water saturation of the filling medium on the dissolution and diffusion of CO₂ and N₂ under real reservoir conditions. The experimental results indicated that the water inside the filling medium was increased from being bound water to being fully saturated, the pressure drop of CO₂ and N₂ was 41.62% and 43.22% and the solubility drop was 40.16% and 42.50% respectively, and the diffusion coefficients were both reduced by an order of magnitude; the existence of formation water in the filling medium reduces the dissolution, diffusion and mass transfer capacities of the injected gas; the physical properties of the formation fluid and the difference between the formation fluid and the porous medium were the main influencing factors. This study provides a reference for the gas injection development of residual oil in fractured-vuggy reservoir in high water-cut stage and the optimization of relevant parameters.

Key words: fracture-cavity reservoir, filling medium, water saturation, gas dissolution and diffusion, CO₂, N₂